1. Field of the Invention
The present invention relates to a pixel structure of an active organic light emitting diode, and more particularly to a pixel structure of an active organic light emitting diode having lightly doped drain regions.
2. Description of the Related Art
By the advance of technology, a variety of computers, cellular phones, PDA, digital cameras, etc. have been developed. In these electronic products, displays are very essential thereto. Because of their small size, weight and low power consumption, flat panel displays have been widely used. In flat panel displays, organic light emitting diode displays have wide view angle, contrast of color, lower weight, high responsive speed and low costs, and are adapted to be used in electronic clocks, cellular phones, PDA, digital cameras, etc. Initially, the organic light emitting diodes are passive-drive diodes. Because the luminescence and service life of these passive diodes will decay by the size and resolution of the displays, organic light emitting diode displays having active-drive diodes are developed.
Please referring to FIG. 1, it is a schematic drawing showing a prior art pixel structure of an organic light emitting diode. The pixel structure of an organic light emitting diode 100 comprises a data-line 104, a scan-line 102, a switch thin film transistor 110, a control thin film transistor 120, a capacitor 130 and an organic light emitting diode 140. The gray level of the pixel structure of an organic light emitting diode 100 is determined by the voltage of the data-line 104. When the scan-line 102 turns on the switch thin film transistor 110, the voltage of the data-line can control the gate terminal (not shown) of the control thin film transistor 120 through the switch thin film transistor 110 for supplying current to the light emitting diode 140 thereby generating different gray level. When the switch thin film transistor 110 is turned on, the capacitor 130 is charged for reserving voltage. When the switch thin film transistor 110 is turned off, the capacitor 130 is discharged for maintaining on-state of the control thin film transistor 120. Therefore, the light emitting diode 140 can provide the same brightness.
Please referring to FIG. 1, the switch thin film transistor 110 and the control thin film transistor 120 can be, for example, an amorphous-silicon thin film transistor or a poly-silicon thin film transistor. Compared with the amorphous-silicon thin film transistor, the poly-silicon thin film transistor has low power consumption and high electron mobility. Although early poly-silicon thin film transistors are processed by a high-temperature method, low-temperature poly-silicon thin film transistors have gradually the high-temperature diodes and become the mainstream in this filed.
When the poly-silicon thin film transistor serves as the switch thin film transistor, leakage current still exists in the channel region even if the switch thin film transistor is turned off. The voltage maintained by the capacitor, therefore, drops and affects the stability of the organic light emitting diode. FIG. 2 is an I-V curve of a prior art poly-silicon thin film transistor. From the curve, the Kink effect arises when the voltage is within the saturation region. It means that the current from the control thin film transistor varies with the driving voltage and cannot maintain at a fixed value. Accordingly, the reliability of the control thin film transistor becomes worse and the luminescence of the diode is affected.